Detecting and characterizing shielded fissionable material (e.g., Special Nuclear Material (SNM)) is a difficult problem. Conventional passive methods rely on spectroscopy of low-energy (less than 500 keV) gamma rays (i.e., photons) from natural decay, but this approach is not suitable when thick shielding may be present. For example, the attenuation of 500 keV gamma rays is such that only about 20% penetrate 2.54 cm of steel shielding, and only about 1% penetrate the same thickness of lead. Higher energy gamma rays can more easily penetrate shielding and, furthermore, increase the detectability of uranium and plutonium isotopes by detecting reaction products of photofission. A means capable of generating a sufficient intensity of high energy photons is needed.
High-energy gamma rays can be produced via (relatively) low energy nuclear reactions. Table 1 summarizes various possible reactions, indicating the minimum incident proton kinetic energy required to initiate the nuclear reaction, the specific reaction concerned, the energy of the emitted gamma ray obtained from the reaction, and the cross-section of the reaction (specified in millibarns). In particular, two reactions for proton induced gamma ray generation are considered (indicated in bold type). One reaction involves 11B(p, γ)12C, i.e., the conversion of boron-11 to carbon-12 by nuclear absorption of a proton, emitting a gamma (γ) ray. At proton energy of 163 keV, the reaction produces an exit gamma ray of 11.7 MeV, with a cross-section of 0.157 millibarns (mb), which is a measure of the “efficiency” of producing the high energy gamma rays. This reaction is nominally referred to as a “12 MeV” reaction, and protons may conventionally be accelerated to approximately 180 keV or higher to assure a substantial production of 12 MeV gammas rays. A second reaction of interest (19F(p, αγ)16O) requires a higher proton energy of 340 keV incident on fluorine-19 to emit an alpha particle (α) and a 6.1 MeV gamma ray, converting the fluorine-19 to oxygen-16, with a reaction cross section of 160 mb. This reaction is nominally referred to as a “6 MeV” reaction, and protons may conventionally be accelerated to approximately 360 keV or higher to assure a sufficient production of 6 MeV gamma rays.
TABLE 1Proton EnergyGamma EnergyCross Section[keV]Reaction[MeV][mb]16311B(p, γ)12C11.70.15722419F(p, αγ)16O6.1, 6.9, 7.10.23309Be(p, γ)10B5.2, 6.2, 6.9—34019F(p, αγ)16O 6.11604417Li(p, γ)8Be12.2, 14.7, 17.66Handbook of Modern Ion Beam Materials Analysis, p. 575 (1995)
FIG. 1 shows a concept for a prior art nuclear reaction gamma ray generator 100, using accelerated protons to impact a target. As shown in FIG. 1, the gamma ray generator 100 shows a plasma generator chamber 105 fed by a gas line 106 at a first pressure suitable for inducing a plasma 110 containing protons or other ionic species. An intermediate second pressure chamber 112 is held at a second pressure by a first vacuum pump 115. A puller electrode 120 with an aperture 125 has an applied voltage biased to draw the positively charged protons (or other positive ions) from the plasma generator chamber 105 into an accelerator chamber 130. The accelerator chamber 130 includes an ion beam accelerator column 140, and a target 150 (containing, for example, boron), where the accelerator chamber 130 is pumped by a second vacuum pump 135 to maintain a third pressure in the accelerator chamber 130 and the accelerator column 140 that is lower than the second pressure, and where the target 150 is maintained at a high negative potential to accelerate the protons to at least 163 keV.
A first vacuum pump 115, coupled to the plasma generation chamber 105 via a gas How limiting plasma aperture 111, maintains a suitable plasma generation pressure at the first pressure in the plasma generator 105 due to gas inflow at the gas line 106 while lowering the pressure at the intermediate second pressure chamber 112. The intermediate second pressure decreases the ion losses due to ion neutralization allowing highly efficient ion transport to the accelerator chamber 130. A second vacuum pump 135 coupled to the accelerator chamber 130 maintains a further lower pressure to provide a satisfactory mean-free-path for the protons to reach the target without substantial loss of energy through collisions along the acceleration path with neutral gas molecules and ensures corona and arcing free operation of the accelerator column 140.
FIG. 2 shows a prior art implementation of a gamma ray generator tube (GT) 200 for the 11B(p, γ)12C reaction that produces 11.7 MeV gamma rays (hereinafter referred to, for convenience as a “12 MeV GT”). The gamma ray GT 200 of FIG. 2 is an implementation of the key features of the gamma ray generator 100 with associated RF plasma induction electronics, high voltage cabling, feed through and insulation and pressure vessel illustrated. Because the minimum proton energy threshold required to produce this reaction in boron is 163 keV, proton acceleration to a higher energy, e.g., 180 keV is commonly employed to guarantee a sufficient production of gamma rays. There are three key component comprising this implementation of a GT: (1) a source of protons, (2) a proton accelerator to achieve substantially 180 keV, and (3) a target containing boron to supply the nuclear conversion reaction.
Proton production may be obtained by plasma generation in a working gas of hydrogen in an RF-induction cavity, i.e., the plasma generator section 105 of FIG. 1. Referring to FIGS. 1 and 2, Plasma generation may conventionally require a gas pressure on the order of 5 10×10−3 Torr. The accelerator column (e.g., the accelerator column 140 of FIG. 1) requires a low enough pressure to function without discharge arcing, which is commonly lower than the plasma 110 gas pressure. Reducing the pressure for satisfactory operation of the accelerator may be done in stages. In the intermediate second pressure chamber 112, the pump 115 may reduce the pressure, due to the flow limiting plasma aperture 111 to approximately 10−4 to 10−5 Torr. The puller electrode 120 has an aperture that is small enough and includes a long channel to limit gas flow into the accelerator column 140. The puller electrode 120 is negatively biased, and therefore functions as an ion extraction aperture to draw positive protons into the accelerator chamber 130. The long channel aperture separates the intermediate pressure second chamber 112 from the accelerator column 140 in the accelerator chamber 130, and a second pumping stage (e.g., the second vacuum pump 135) then lowers the pressure in the accelerator column 140 to about 10−6 to 10−7 Torr, or less. In other configurations, the respective pressures in the two chambers may be other than the exemplary pressures suggested here.
Once in the accelerator chamber 130, the required kinetic energy for the desired nuclear reaction must be achieved. In the case of the boron reaction 11B(p, γ)12C a single pair of electrodes forming an axial beam of protons may be sufficient to obtain the ˜180 keV required to produce 12 MeV gamma rays, however the current may not be sufficient to produce gamma rays in sufficient quantity, as determined by the cross-section. In addition, any effort to make the system more compact makes a single pair electrode beam accelerator more difficult to achieve, as the higher electric field gradient under the existing pressure conditions may not be stable, leading to arc discharge. A conventional target material source of boron nuclei is LaB6. For the 19F(p, αγ)16O reaction, which requires 340 keV, this is correspondingly more difficult. Since fluorine occurs naturally as a gas, a solid fluorinated compound must be used.
It would therefore be advantageous to provide an ion accelerator that reduces the electric field gradient to prevent arc discharge in the accelerator region while providing the kinetic energy needed to produce the desired nuclear reaction.